This invention relates to a piston ring with an improved wear resistance and scuff resistance, and in particular relates to a piston ring having an electroplated layer comprising dispersed particles in a matrix of nick-cobalt-phosphorous alloy.
In the prior art it is per se known to utilize hard chrome plating in order to improve the wear resistance of a piston ring which undergoes sliding motion at high speed in a cylinder bore, but the disadvantages of doing this are, not only that plating this hard chrome plating onto to the piston ring takes a long time, but also the wear resistance, the scuff resistance, and the corrosion resistance of the piston ring suffer from some problems when the piston ring is used in an engine which burns a leaded gasoline or is subjected to high load operation.
In order to solve such problems, there is a per se known plating process, popularly called the "composite plating process" or the "dispersion plating process", according to which wear resistant particles such as nitrides, carbides, and oxides of a metal are included in a matrix of matrix metal containing nickel and phosphorous, for instance.
According to such a plating process, by appropriately selecting the material, the size, and the dispersion amount of the particles to be dispersed, it is possible to form a plated layer which is superior in wear resistance, scuff resistance, and corrosion resistance, and this process has nowadays started to be used on cylinders and piston rings.
The wear resistance particles which are dispersed in such a composite plated layer improve the wear resistance of the plated layer, but if the dispersion amount is excessive the mechanical strength of the plated layer may be reduced. On the other hand, the addition of phosphorous to the matrix of the composite plated layer can improve the hardness, the wear resistance, and the corrosion resistance of the plated layer when it is subjected to heat processing, but when the content of the phosphorous is excessive the matrix may become more brittle, and the impact strength of the plated layer may be reduced, thereby giving rise to the problems of the plated layer peeling off during use.
Furthermore, a conventional composite plated layer has problems such as wearing out the mating member along which it slides, which in the case of a piston ring is of course the cylinder bore in which it is fitted.